The general objective of this research is to analyze and describe certain aspects of the mechanics of breathing with emphasis on the pressure-flow relationships of the breathing system considered as a reflection of the force-velocity properties of the respiratory muscles. By studying a variety of breathing maneuvers and by using external loads of different types and magnitudes information regarding the manner in which the muscular power source interacts with the loads imposed upon it will be obtained. Airway opening pressure and in some cases esophageal pressure will be measured simultaneously with respiratory air flow or rate of volume change. Maximal and submaximal breathing efforts will be studied. Elastic, resistive, and inertial loads as well as complete airway occlusion will be employed.